In most electrically variable transmissions, an internal combustion power plant is combined with an electrical source of power, such as batteries, to operate an electrically variable transmission. The electrically variable transmission (EVT) generally includes at least one motor/generator and usually two motor/generators, which are operable connected through planetary gear mechanisms to a transmission output. The power supplied between a transmission input and the output from both a prime mover, such as an internal combustion engine, and the motor/generator units.
As a general rule with such EVTs, a power take-off unit, which drives accessories, such as fans and pumps, receives power from the internal combustion engine. In at least one EVT, shown in U.S. Ser. No. 10/364,171, filed Feb. 11, 2003, and assigned to the assignee of the present application, a fuel cell and batteries are the sole prime movers for the EVT. In that patent application, the power take-off unit is driven at a constant speed during high transmission output speed. The power take-off unit is driven at variable speeds during lower transmission output speeds. The speed of the power take-off unit is minimum at zero vehicle speed.